Your outer barrel is the barrel that would reprisent the true barrel of your gun (I.E. the part you see). Your inner barrel is within the outer barrel (surprised?) and is usually made of brass. The inner barrel is the one in which the BB will actually travel down. The resaon people swap or upgrade barrels can vary, but is usually to change the lentgh, or to change the bore. The bore of a stock barrel is roughly 6.08mm. With many BB brands having tolerances of +-.02mm means that the barrel can be of even lower diameter bore, which increases the velocity, and is in many respects a very easy upgrade.